Retro-inverso C-terminal hexapeptide analogues of substance P

ABSTRACT

New retro-inverso peptides and peptide derivatives in the form of analogues of C-terminal hexapeptide fragments of Substance P, which are pharmacologically active, possess prolonged action with time, and are of general formula (I): ##STR1## they being useful as vasedilators.

This invention relates to new peptides and peptide derivatives in theform of analogues of C-terminal hexapeptide fragments of Substance P,which are pharmacologically active and useful as vasodilators. SubstanceP, an undecapeptide of Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH₂sequence, which is considered to be a neuromodulator andneurotransmitter of the central nervous system, is probably involvedboth in the neurotransmission of pain sensations and in the sensoryaxonal reflexes [Celander O, et al, Acta Physiol. Scand. 29, 359 (1953);Haensler G. et al., Naunyn-Schiemedeberg's Arch. Pharmacol., 314, 111(1980)].

In addition it is one of the most powerful of known vasodilators,possessing the property in the peripheral nervous system of contractingthe smooth muscle. [Henbeck F. et al., Int. Rev. Neurobiol., 4, 159(1962); Chernuk A. M. et al., Exp. Biol. Med. 90, 1165 (1980); SchrauwenE. et al., Phlungers Arch. Eur. J. Physio. 986, 281 (1980)]. It is knownthat the various activities of Substance P are also preserved in itsC-terminal segments, in particular in the C-terminal hexa, hepta andoctapeptide fragments.

In this respect, structure-function studies carried out on a series ofpartial sequences and fragments of Substance P, prepared by chemicalsynthesis, have shown that the Substance P receptor interacts best withthe natural C-terminal hexa and heptapeptide sequences, and thatextending the chain beyond the C-terminal heptapeptide produces effectsof negligible importance.

However, both Substance P and its C-terminal hexa, hepta and octapeptidefragments are rapidly degraded by numerous proteolytic enzymes.

In particular, a highly specific membrane enzyme isolated from the brainhydrolyses three different Substance P bonds, namely Gln⁶ -Phe⁷, Phe⁷-Phe⁸ and Phe⁸ -Gly⁹ [Hec G. M. et al., Eur. J. Biochem. 114, 315(1981)].

The extreme lability of Substance P and its C-terminal fragments towardsenzymes makes their use in pharmacology of little advantage andproblematical.

Attempts have been made to stabilise the molecule by using the methodsdescribed in Biochem. Biophys. Res. Comm. 90. 347 (1979) and in Eur. J.Biochem. 114 329, (1981) respectively. However, in the case of SubstanceP, a loss of power of the molecule has been observed.

It has now been found that by applying the criterion of retro-inversionof suitable peptide bonds to the C-terminal hexapeptide fragments ofSubstance P, it is possible to obtain metabolically stable peptides withunaltered biological activity.

The present invention relates to new peptides and peptide derivatives inthe form of analogues of the hexapeptide fragments of Substance P inwhich two of the peptide bonds which have proved to be most susceptibleto endopeptidase action have been simultaneously inverted. In particularwe have retro-inverted the Phe⁷ -Phe⁸ and Phe⁸ -Gly⁹ bonds of thesequence of the C-terminal hexapeptide fragment of Substance P.

The inversion of two adjacent peptide bonds involves the simultaneoustransformation of the three amino acids of the retro-inverted peptidesegment. In order to maintain the biological activity of said peptideunaltered, it is necessary to effect the inversion in such a manner thatthe three-dimensional orientation of the peptide side-chains ismaintained.

The amino acid residue closest to the N-terminus is transformed into aderivative of gem-diaminoalkylmethane type, that closest to theC-terminus is transformed into a malonyl or 2-substituted malonylderivative, and the amino acid residue between the two retro-invertedbonds possesses D configuration.

The malonyl or 2-substituted malonyl residue is incorporated into thepeptide chain as described by Goodman M. in "Perspectives in PeptideChemistry", A. Eberle, R. Geiger and T. Wieland, Editors, Karger, Basel1980 p. 283, whereas the gem-diaminoalkylmethane derivative isintroduced as described in Italian Pat. Appln. No. 25755A/81 filed onDec. 22, 1981.

The retro-inverse peptides according to the present invention are thosecorresponding to general formula (I): ##STR2## in which P is a hydrogenatom, a linear or branched aliphatic alkyl group with 1-6 carbon atoms,or a saturated or unsaturated linear or branched chain aliphatic acylgroup such as formyl, acetyl, propionyl, n-butyryl, isobutyryl,n-valeryl, isovaleryl, hexanoyl, isohexanoyl, heptanoyl, octanoyl,crotonoyl, methacryloyl, acryloyl; or a substituted acyl group such ashydroxyacetyl, 2-hydroxypropionyl, 3-hydroxypropionyl, aminoacetyl,4-hydroxyphenylacetyl, 4-hydroxyphenylpropionyl, 2-aminopropionyl,3-aminopropionyl, O-ethyl-malonyl, ethoxyformyl, methoxyformyl,3-methoxypropionyl, 3-ethoxypropionyl, chloroacetyl, dichloroacetyl,2-chloropropionyl, 3-chloropropionyl, 2,3-dichloropropionyl,bromoacetyl, 4-hydroxy-3,5-diiodophenylacetyl, 3-oxobutyryl,3-oxovaleryl, 4-oxovaleryl, methylthioacetyl, 3-methylthiopropionyl,ethylthioacetyl, 3-ethylthiopropionyl, nicotinoyl, 4-aminobutyryl,N.sup.α -[(1-(9-adenyl)β-D-ribofuranuronosyl], N.sup.α-[(1-(9-hypoxanthyl)-β-D-ribofuranuronosyl]; or a group such asbenzyloxycarbonyl, tert-butyloxycarbonyl, tert-amyloxycarbonyl,isobornyloxycarbonyl, adamantyloxycarbonyl, or chloro ornitro-substituted benzyloxycarbonyl;

R¹ is a residue of methionine, methioninesulphoxide, methioninesulphone, selenomethionine, leucine, norleucine, valine or norvaline;

R² is a residue of leucine, norleucine, valine, norvaline, alanine orisoleucine;

R³ is a hydrogen atom or methyl;

R⁴ is an amino acid residue of D configuration such as phenylalanine,tryptophan, tyrosine, valine, norvaline, leucine, norleucine,isoleucine, serine or derivatives, threonine or derivatives, histadineor derivatives, methionine, methionine-S-methyl, methionine sulphone,arginine or derivatives, lysine or derivatives, ornithine orderivatives, 2,4-diaminobutyric acid or derivatives,2,3-diaminopropionic acid or derivatives, glutamic acid or aspartic acidor their suitable derivatives;

R⁵ is a hydrogen atom or the side-chain of amino acids such asphenylalanine, tyrosine, 4-chlorophenylalanine, O-benzyltyrosine (ortheir acetyl, cyclopentyl, tert-butyl-oxycarbonyl or4-hydroxyphenylacetyl derivatives);

R⁶ is an amino acid residue such as glutamine or derivatives,pyroglutamic acid, alanine, tyrosine, lysine or derivatives, proline,N-formyl-proline, β-alanine, N-acetyl-β-alanine, glycine,desaminophenylalanine, desaminoaspartic acid, methyldesaminoasparticacid, or glutamic acid esters represented by general formula (II):##STR3## in which X is methyl, ethyl, methoxyethyl, methoxy(ethoxy)_(n)ethyl where n=1, 2 or 3.

Unless otherwise specified, each amino acid is of L form. A hexapeptidederivative of general formula (I) is synthesised by condensation,induced generally by DCC+HOBt, of a N,N'-diacylatedgem-diaminoalkylmethane residue of general formula (III): ##STR4## inwhich P, R⁶, R⁵ and R⁴ have the aforesaid meanings and in which theamino, hydroxyl, carboxyl, carboxyamido, indole, imidazole, guanidinoand mercaptide functions are suitably protected if present in P, R⁶, R⁵or R⁴, with a peptide fragment of general formula IV: ##STR5## where R¹,R² and R³ have the aforesaid meanings.

The synthesis of the diacylated gem-diaminoalkylmethane residue ofgeneral formula III and of the peptide fragment of general formula IV isconducted by known methods.

The identity of the products obtained is demonstrated by nuclearmagnetic resonance spectroscopic analysis.

Product purity is demonstrated by reverse phase high pressurechromatography analysis (RP-hplc) using 0.01M ammonium acetate/MeCN asthe eluent system, or thin layer chromatography analysis in silica gelusing the following eluent systems: n-butanol/acetic acid/water (4:1:1);chloroform/methanol/acetic acid (85:10:5).

The following abbreviations are used in the synthesis descriptions:Boc=tert-butyloxycarbonyl; Z=benzyloxycarbonyl; MeO=methyl ester;MeOH=methyl alcohol; EtOH=ethyl alcohol; THF=tetrahydrofuran; EtAc=ethylacetate; Et₂ O=ethyl ether; MeCN=acetonitrile;DMF=N,N-dimethylformamide; DCC=N,N'-dicyclohexylcarbodiimide;DCU=N,N'-dicyclohexylurea; HOBt=N-hydroxybenzotriazole;TIB=1,1-bis(trifluoroacetoxy)iodobenzene; NMM=methylmorphaline; gPhe=##STR6## mGly=CO--CH₂ --CO; mAla= ##STR7## The pharmacological activityof the retro-inverse analogues of the present invention was verified bymeasuring the contraction of the isolated ileum of the guinea pig, asdescribed by Rossel et al. in "Substance P" V. S. von Euler and B.Pernow Editors, Raven Press, New York 1977 p. 83, and by the in vivoevaluation of the main hemodynamic parameters.

It was found that the in vitro activity of said analogues is about 50times less than that of Substance P.

In contrast, in vivo they demonstrate the same type of action asSubstance P, ie a gradual hypotensive action prolongued with time, and areduction in carotid arterial flow.

The subject matter and object of the invention will be more apparentfrom a reading of the following example, which is given only forillustrative purposes and must in no way be considered as limitative ofthe invention itself.

EXAMPLE Synthesis oftert-butyloxycarbonyl-alanyl-gemphenylalanyl-D-phenylalanyl-malonyl-leucyl-methionineamide:Synthesis of tert-butyloxycarbonyl-leucylmethionine methyl esterBoc-Leu-Met-OMe

1.0 equivalent of Boc-Leu is dissolved in anhydrous THF, and 1.0equivalent of NMM and 1.1 equivalents of isobutyl chloroformate areadded to the solution, which is cooled to -15° C. and maintained in anitrogen atmosphere.

After two minutes, a solution prepared by dissolving 1.0 equivalent ofHCl.Met-OMe and 1.0 equivalent of NMM in DMF is added. During theadditions, the temperature is checked to ensure that it does not exceed-10° C. Having verified the disappearance of HCl.Met-OMe, the reactionis blocked by evaporating the mixture to dryness, the residue is takenup in EtAc and washed with a 5% bicarbonate solution, water, a 5% citricacid solution, and water.

The EtAc solution is then dried over magnesium sulphate, and the productis obtained by crystallisation, by suitably adding 30°-50° C. petroleumether.

M.P.=102°-104° C.

[α]₂₂°⁵⁸⁹ =-36.1° (C=1.0 in DMF)

Elementary analysis for C₁₇ H₃₂ N₂ O₅ S: Theoretical: C 54.23; H 8.57; N7.44. Found: C 54.10; H 8.49; N 7.39.

Chromatography analysis (tlc and hplc) shows no presence of impurities,and the ¹ H n.m.r. spectrum confirms the molecular structure.

Synthesis of tert-butyloxycarbonyl-leucylmethionineamide Boc-Leu-Met-NH₂

1.0 equivalent of Boc-Leu-Met-OMe is dissolved in anhydrous MeOH, andanhydrous ammonia is passed for 30 minutes into this solution, cooled to-5° C.

The solution, contained in a hermetically sealed vessel, is keptovernight at ambient temperature, after which the product is obtained incrystalline form by adding a volume of water equal to about 30% of thevolume of MeOH.

M.P.=158°-160° C.

[α]₂₂°⁵⁸⁹ =-35.4° (C=1.0 in DMF).

Elementary analysis for C₁₆ H₃₁ N₃ O₄ S: Theoretical: C 53.16; H 7.33; N11.63; Found: C 53.03; H 7.23; N 11.50.

Chromatography analysis (tlc and hplc) shows no presence of impurities,and the ¹ H n.m.r. spectrum confirms the molecular structure.

Synthesis of malonylleucylmethionineamide methyl esterMeO-mGly-Leu-Met-NH₂

1.0 equivalent of methyl monomalonate is dissolved in CH₂ Cl₂, and 1.5equivalents of HObt dissolved in DMF and 1.1. equivalents of DCCdissolved in CH₂ Cl₂ are added to the solution cooled to 0° C. After 20minutes, 1.0 equivalent of HCl.Leu-Met-NH₂ (obtained by removing thetert-butyloxycarbonyl from Boc-Leu-Met-NH₂ using 4.5N HCl in EtAc) isadded to the cold mixture, followed by 1.1 equivalents of NMM. The icebath is removed after about one hour, and having checked thedisappearance of the hydrochloride, the reaction mixture is filtered,the precipitated dicyclohexylurea is washed with portions of THF, andthe resultant solution and wash liquors are evaporated to dryness. Theresidue obtained is washed with small volumes of 5% sodium bicarbonate,water, 5% citric acid and water, and is then dried over P₂ O₅.

The product is crystallised from DMF/Et₂ O.

M.P.=184°-185° C.

[α]₂₂°⁵⁸⁹ =-33.6 (C=0.87 in DMF)

Elementary analysis for C₁₅ H₂₇ N₃ O₅ S: Theoretical: C 49.86; H 7.48; N11.66; Found: C 49.75; H 7.43; N 11.60.

Chromatography analysis (tlc and hplc) shows no presence of impurities,and the ¹ H n.m.r. spectrum confirms the molecular structure.

Synthesis of malonylleucylmethionineamide HO-mGly-Leu-Met-NH₂

1.0 equivalent of MeO-mGly-Leu-Met-NH₂ is dissolved in MeOH, and 3equivalents of a 3M aqueous solution of NaOH are added to the solution.

Having verified the disappearance of the starting ester, the methanol isdiluted with water, then eliminated, and the residual solution acidifiedto pH 2 with concentrated HCl, and extracted repeatedly with EtAc. Theextracts are pooled, dried with magnesium sulphate and evaporated todryness. The product is crystallised from dioxane/30°-50° C. petroleumether.

M.P.=136°-138° C. (dec.)

[α]₂₂°⁵⁸⁹ =-38.9 (C=1.3 in DMF)

Elementary analysis for C₁₄ H₂₅ N₃ O₅ S: Theoretical: C 48.41; H 7.20; N12.10; Found: C 47.12; H 7.91; N 12.00.

Chromatography analysis (tlc and hplc) shows no presence of impurities,and the ¹ H n.m.r. spectrum confirms the molecular structure.

Synthesis of tert-butyloxycarbonyl-alanylphenylalanine methyl esterBoc-Ala-Phe-OMe

1.0 equivalent of Boc-Ala is dissolved in anhydrous THF, and 1.1equivalents of NMM and 1.1 equivalents of isobutyl chloroformate areadded to the solution cooled to -15° C. and maintained in a nitrogenatmosphere. After two minutes, 1.0 equivalent of HCl.Phe-OMe and 1.0equivalent of NMM are added.

During the addition of the isobutyl chloroformate and the HCl.Phe-OMe,the temperature is checked to ensure that it does not exceed -10° C.Having verified the disappearance of HCl.Phe-OMe, the reaction issuspended by evaporating the solvent mixture to dryness. The residue istaken up in EtAc and washed with 5% sodium bicarbonate, water, 5% citricacid and water. The organic solution is dried over magnesium sulphate.The product is obtained by crystallisation from EtAc/petroleum ether.

M.P.=82°-84° C.

[α]₂₂°⁵⁴⁶ =-22.97 (C=0.91 in MeOH)

Elementary analysis for C₁₈ H₂₆ N₂ O₅ : Theoretical: C 61.69; H 7.48; N8.00; Found: C 61.08; H 7.42; N 7.95.

Chromatography analysis (tlc and hplc) shows no presence of impurities,and the ¹ H n.m.r. spectrum confirms the molecular structure.

Synthesis of tert-butyloxycarbonyl-alanylphenylalanine amideBoc-Ala-Phe-NH₂

1.0 equivalent of Boc-Ala-Phe-NH₂ is dissolved in MeOH, and anhydrousammonia is passed for 30 minutes into this solution cooled to -5° C. Thesolution, contained in a hermetically sealed vessel, is kept overnightat ambient temperature, after which the solution is evaporated todryness. The solid residue is washed under hot conditions with Et₂ O andfiltered.

M.P.=167°-168° C.

[α]₂₂°⁵⁴⁶ =-43.46 (C=1.37 in MeOH).

Elementary analysis for C₁₇ H₂₅ N₃ O₄ : Theoretical: C 60.87; H 7.51; N12.52; Found: C 60.83; H 7.47; N 12.50.

Chromatography analysis (tlc and hplc) and ¹ H n.m.r. show no presenceof impurities, and confirm the molecular structure.

Synthesis of tert-butyloxycarbonyl-alanylgemphenylalanine hydrochlorideBoc-Ala-gPhe.HCl

1.0 equivalent of Boc-Ala-Phe-NH₂ is dissolved in a 2:3 v/v water:MeCNmixture. 1.2 equivalents of TIB dissolved in MeCN are added to thesolution at ambient temperature under agitation.

An inert gas is bubbled through the reaction mixture in order tofacilitate removal of the CO₂ developed during the reaction. Havingverified the disappearance of Boc-Ala-Phe-NH₂, the reaction is suspendedand the reaction solvent is evaporated to dryness. 1.0 equivalent of0.4N HCl in EtAc is added to the residue after taking up in EtAc. Onthen adding Et₂ O, a white precipitate is obtained which is isolated byfiltration. The product is obtained by crystallising the precipitatewith EtOH/Et₂ O.

M.P.=135.5°-137° C.

[α]₂₂°⁵⁴⁶ =-54.15 (C=1.12 in MeOH).

Elementary analysis for C₁₆ H₂₆ N₃ O₃ Cl: Theoretical: C 55.88; H 7.62;N 12.22; Found: C 55.81; H 7.58; N 12.07.

Chromatography analysis (tlc and hplc) shows no presence of impurities,and the ¹ H n.m.r. spectrum confirms the molecular structure.

Synthesis oftert-butyloxycarbonyl-alanylgemphenylalanyl-D-benzyloxycarbonyl-phenylalanineBoc-Ala-gPhe-D-Z-Phe

1.0 equivalent of Z-D-Phe are dissolved in DMF, and 1.1 equivalents ofHOBt and 1.1 equivalents of DCC dissolved in DMF are added to thesolution cooled to 0° C. After 20 minutes, 1.0 equivalent ofBoc-Ala-gPhe.HCl and 1.0 equivalent of NMM dissolved in DMF are added tothe cold mixture. The ice bath is removed after one hour.

After checking the disappearance of Boc-Ala-gPhe.HCl, the reaction issuspended, the precipitated DCU is filtered off, and the reactionsolvent evaporated to dryness. The residue is taken up in CHCl₃ andwashed with 5% sodium bicarbonate, water, 5% citric acid and water. Theorganic phase is dried over magnesium sulphate and evaporated todryness. The required product is obtained by precipitation fromTHF/petroleum ether.

M.P.=195°-197° C.

[α]₂₂°⁵⁴⁶ =8.53° (C=1.48 in DMF).

Elementary analysis for C₃₃ H₄₀ N₄ O₆ : Theoretical: C 67.32; H 6.85; N9.51; Found: C 67.02; H 6.79; N 8.45.

Chromatography analysis (tlc and hplc) and ¹ H n.m.r. show no presenceof impurities, and confirm the molecular structure.

Synthesis oftert-butyloxycarbonylalanylgemphenylalanyl-D-phenylalanylmalonylleucylmethionineamide Boc-Ala-gPhe-D-Phe-Gly-Leu-Met-NH₂

1.0 equivalent of HO-mGly-Leu-Met-NH₂ is dissolved in THF, and 1.1equivalents of HOBt dissolved in DMF and 1.1 equivalents of DCCdissolved in THF are added to the solution cooled to 0° C. After 20minutes, 1.0 equivalent of Boc-Ala-gPhe-D-Phe (obtained by removing thebenzyloxycarbonyl group from Boc-Ala-gPhe-D-Z-Phe by catalytichydrogenation with 10% Pd on carbon) and 1.1 equivalents of NMMdissolved in DMF are added to the cold mixture.

After 20 hours, and having verified the disappearance ofBoc-Ala-gPhe-D-Z-Phe, the reaction mixture is filtered. The resultantsolution is evaporated to dryness and the solid residue is washed with5% sodium bicarbonate, water, citric acid, water and methanol.

M.P.=248°-250° C.

[α]₂₂°⁵⁴⁶ =-25.14 (C=1.36 in DMF).

Elementary analysis for C₃₉ M₅₇ N₇ O₈ S: Theoretical: C 59.74; H 7.33; N12.51; Found: C 59.58; H 7.09; N 12.45.

Chromatography analysis (tlc and hplc) and ¹ H n.m.r. show no presenceof impurities and confirm the molecular structure.

We claim:
 1. Compounds corresponding to general formula I: ##STR8## inwhich P is a hydrogen atom, a linear or branched aliphatic alkyl groupwith 1-6 carbon atoms, or a saturated or unsaturated linear or branchedchain aliphatic acyl group such as formyl, acetyl, propionyl, n-butyryl,isobutyryl, n-valeryl, isovaleryl, hexanoyl, isohexanoyl, heptanoyl,octanoyl, crotonoyl, methacryloyl, acryloyl; or a substituted acyl groupsuch as hydroxyacetyl, 2-hydroxypropionyl, 3-hydroxypropionyl,aminoacetyl, 4-hydroxyphenylacetyl, 4-hydroxyphenylpropionyl,2-aminopropionyl, 3-aminopropionyl, O-ethyl-malonyl, ethoxyformyl,methoxyacetyl, 3-methoxypropionyl, 3-ethoxypropionyl, chloroacetyl,dichloroacetyl, 2-chloropropionyl, 3-chloropropionyl,2,3-dichloropropionyl, bromoacetyl, 4-hydroxy-3,5-diiodophenylacetyl,3-oxobutyryl, 3-oxovaleryl, 4-oxovaleryl, methylthioacetyl,3-methylthiopropionyl, ethylthioacetyl, 3-ethylthiopropionyl,nicotinoyl, 4-aminobutyryl, N.sup.α-[(1-(9-adenyl)β-D-ribofuranuronosyl)], N.sup.α-[(1-(9-hypoxanthyl)-β-D-ribofuranuronosyl]; or a group such asbenzyloxycarbonyl, tert-butyloxycarbonyl, tert-amyloxycarbonyl,isobornyloxycarbonyl, adamantyloxycarbonyl, or chloro ornitro-substituted benzyloxycarbonyl;R¹ is a residue of methionine,methioninesulphoxide, methionine sulphone, selenomethionine, leucine,norleucine, valine or norvaline; R² is a residue of leucine, norleucine,valine, norvaline, alanine or isoleucine; R³ is a hydrogen atom ormethyl; R⁴ is an amino acid residue of D configuration such asphenylalanine, tryptophan, tyrosine, valine, norvaline, leucine,norleucine, isoleucine, serine or derivatives, threonine or derivatives,histadine or derivatives, methionine, methionine-S-methyl, methioninesulphone, arginine or derivatives, lysine or derivatives, ornithine orderivatives, 2,4-diaminobutyric acid or derivatives,2,3-diaminopropionic acid or derivatives, glutamic acid or aspartic acidor their suitable derivatives; R⁵ is a hydrogen atom or the side-chainof amino acids such as phenylalanine, tyrosine, 4-chlorophenylalanine,O-benzyltyrosine (or their acetyl, cyclopentyl, tert-butyl-oxycarbonylor 4-hydroxyphenylacetyl derivatives); R⁶ is an amino acid residue suchas glutamine or derivatives, pyroglutamic acid, alanine, tyrosine,lysine or derivatives, proline, N-formyl-proline, β-alanine,N-acetyl-β-alanine, glycine, desaminophenylalanine, desaminoasparticacid, methyldesaminoaspartic acid, or glutamic acid esters representedby general formula (II): ##STR9## in which X is methyl, ethyl,methoxyethyl, methoxy(ethoxy)_(n) ethyl where n=1, 2 or
 3. 2. Thepeptide Boc-Ala-gPhe-D-Phe-mGly-Leu-Met-NH₂ in which all the amino acidsare of L configuration.
 3. The peptidecyclopentyl-Ala-gPhe-mGly-Leu-Met-NH₂ in which all the amino acids areof L configuration.
 4. The peptide Glp-gPhe-D-Phe-mGly-Leu-Met-NH₂ inwhich all the amino acids are of L configuration.
 5. The peptideGlp-gPhe-D-Phe-(R,S)mAla-Leu-Met-NH₂ in which all the amino acids are ofL configuration.
 6. The peptide HCO-Pro-gPhe-D-Phe-mGly-Leu-Met-NH₂ inwhich all the amino acids are of L configuration.
 7. The peptideHCO-Pro-gPhe-D-Phe-(R,S)mAla-Leu-Met-NH₂ in which all the amino acidsare of L configuration.
 8. The peptideBoc-Pro-gPhe-D-Phe-mGly-Leu-Met-NH₂ in which all the amino acids are ofL configuration.
 9. The peptide Boc-Pro-gPhe-D-Phe-(R,S)mAla-Leu-Met-NH₂in which all the amino acids are of L configuration.
 10. The peptidecyclopentyl-Pro-gPhe-D-Phe-mGly-Leu-Met-NH₂ in which all the amino acidsare of L configuration.
 11. The peptideCyclopentyl-Pro-gPhe-D-Phe-(R,S)mAla-Leu-Met-NH₂ in which all the aminoacids are of L configuration.
 12. The peptidecyclopentyl-Gln-gPhe-D-Phe-mGly-Leu-Met-NH₂ in which all the amino acidsare of L configuration.
 13. The peptidecyclopentyl-Gln-gPhe-D-Phe-(R,S)mAla-Leu-Met-NH₂ in which all the aminoacids are of L configuration.
 14. The peptidePyr-gPhe-D-Phe-mGly-Leu-Met(O)-NH₂ in which all the amino acids are of Lconfiguration.
 15. The peptide Pyr-gPhe-D-Phe-(R,S)mAla-Leu-Met(O)-NH₂in which all the amino acids are of L configuration.